.\" -*- nroff -*-
.\" Copyright 2013 Los Alamos National Security, LLC. All rights reserved.
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" $COPYRIGHT$
.TH MPI_Pack_external 3 "Jun 10, 2020" "4.0.4" "Open MPI"

.SH NAME
\fBMPI_Pack_external\fP \- Writes data to a portable format

.SH SYNTAX
.ft R

.SH C Syntax
.nf
#include <mpi.h>
int MPI_Pack_external(const char *\fIdatarep\fP, const void *\fIinbuf\fP,
	int \fIincount\fP, MPI_Datatype\fI datatype\fP,
	void *\fIoutbuf\fP, MPI_Aint \fIoutsize\fP,
	MPI_Aint *\fIposition\fP)

.fi
.SH INPUT PARAMETERS
.ft R
.TP 1i
datarep
Data representation (string).
.ft R
.TP 1i
inbuf
Input buffer start (choice).
.TP 1i
incount
Number of input data items (integer).
.TP 1i
datatype
Datatype of each input data item (handle).
.TP 1i
outsize
Output buffer size, in bytes (integer).

.SH INPUT/OUTPUT PARAMETER
.ft R
.TP 1i
position
Current position in buffer, in bytes (integer).

.SH OUTPUT PARAMETERS
.ft R
.TP 1i
outbuf
Output buffer start (choice).
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
MPI_Pack_external packs data into the external32 format, a universal
data representation defined by the MPI Forum. This format is useful
for exchanging data between MPI implementations, or when writing data
to a file.
.sp
The input buffer is specified by \fIinbuf\fP, \fIincount\fP and
\fIdatatype\fP, and may be any communication buffer allowed in
MPI_Send. The output buffer \fIoutbuf\fP must be a contiguous storage
area containing \fIoutsize\fP bytes.
.sp
The input value of \fIposition\fP is the first position in
\fIoutbuf\fP to be used for packing (measured in bytes, not elements,
relative to the start of the buffer). When the function returns,
\fIposition\fP is incremented by the size of the packed message, so
that it points to the first location in \fIoutbuf\fP following the
packed message. This way it may be used as input to a subsequent call
to MPI_Pack_external.
.sp

\fBExample:\fP An example using MPI_Pack_external:
.sp
.nf
	int position, i;
	double msg[5];
	char buf[1000];

	\&...

	MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
	if (myrank == 0) {	/* SENDER CODE */
		position = 0;
		i = 5; /* number of doubles in msg[] */
		MPI_Pack_external("external32", &i, 1, MPI_INT,
		    buf, 1000, &position);
		MPI_Pack_external("external32", &msg, i, MPI_DOUBLE,
		    buf, 1000, &position);
		MPI_Send(buf, position, MPI_BYTE, 1, 0,
		    MPI_COMM_WORLD);
	} else {		/* RECEIVER CODE */
		MPI_Recv(buf, 1, MPI_BYTE, 0, 0, MPI_COMM_WORLD,
		    MPI_STATUS_IGNORE);
		MPI_Unpack_external("external32", buf, 1000,
		    MPI_INT, &i, 1, &position);
		MPI_Unpack_external("external32", buf, 1000,
		    MPI_DOUBLE, &msg, i, &position);
	}

.fi
.SH NOTES
.ft R
The \fIdatarep\fP argument specifies the data format. The only valid
value in the current version of MPI is "external32". The argument is
provided for future extensibility.
.sp
To understand the behavior of pack and unpack, it is convenient to
think of the data part of a message as being the sequence obtained by
concatenating the successive values sent in that message. The pack
operation stores this sequence in the buffer space, as if sending the
message to that buffer. The unpack operation retrieves this sequence
from buffer space, as if receiving a message from that buffer. (It is
helpful to think of internal Fortran files or sscanf in C for a
similar function.)
.sp
Several messages can be successively packed into one packing
unit. This is effected by several successive related calls to
MPI_Pack_external, where the first call provides \fIposition\fP=0,
and each successive call inputs the value of \fIposition\fP that was
output by the previous call, along with the same values for
\fIoutbuf\fP and \fIoutcount\fP. This packing unit now contains the
equivalent information that would have been stored in a message by one
send call with a send buffer that is the "concatenation" of the
individual send buffers.
.sp
A packing unit can be sent using type MPI_BYTE. Any point-to-point
or collective communication function can be used to move the sequence
of bytes that forms the packing unit from one process to another. This
packing unit can now be received using any receive operation, with any
datatype. (The type-matching rules are relaxed for messages sent with
type MPI_BYTE.)
.sp
A packing unit can be unpacked into several successive messages. This
is effected by several successive related calls to
MPI_Unpack_external, where the first call provides \fIposition\fP=0,
and each successive call inputs the value of position that was output
by the previous call, and the same values for \fIinbuf\fP and
\fIinsize\fP.
.sp
The concatenation of two packing units is not necessarily a packing
unit; nor is a substring of a packing unit necessarily a packing
unit. Thus, one cannot concatenate two packing units and then unpack
the result as one packing unit; nor can one unpack a substring of a
packing unit as a separate packing unit. Each packing unit that was
created by a related sequence of pack calls must be unpacked as a unit
by a sequence of related unpack calls.

.SH ERRORS
.ft R
Almost all MPI routines return an error value; C routines as
the value of the function and Fortran routines in the last argument. C++
functions do not return errors. If the default error handler is set to
MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
will be used to throw an MPI::Exception object.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with
MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN
may be used to cause error values to be returned. Note that MPI does not
guarantee that an MPI program can continue past an error.
.sp
See the MPI man page for a full list of MPI error codes.

.SH SEE ALSO
.ft R
.nf
MPI_Pack_external_size
MPI_Send
MPI_Unpack_external
sscanf(3C)

